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MMU RAMROM Mode: Difference between revisions
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Color Computer 3 Memory Management Unit in ROM/RAM | Color Computer 3 Memory Management Unit in ROM/RAM | ||
The | The ROM pages ($3C-$3F) may be written to any of the eight MMU slots. However, unlike RAM pages, the MMU ignores the lower two bits of the value written when the page refers to ROM. Instead, it replaces those bits with the lower two bits of the slot number. | ||
For example, writing $3D to slot 2 of the MMU does not map the second 8k page of ROM to addresses $4000-$5FFF. It maps page $3E (the third ROM page), because: | |||
[[image:ROMRAM.png|center]] | |||
In ROM/RAM mode there are three different ROM configurations available. They are selected by setting bits 1 and 0 of address $FF90 to: | In ROM/RAM mode there are three different ROM configurations available. They are selected by setting bits 1 and 0 of address $FF90 to: | ||
| Line 57: | Line 61: | ||
|- | |- | ||
| $E000 - $FFFF | | $E000 - $FFFF | ||
| Slot 7: $ | | Slot 7: $FFA7, $FFAF | ||
|} | |} | ||
The ROM pages ($3C-$3F) can be written to any of the eight available MMU slots. In all cases the last two bits of the page are ignored by the MMU and substituted by the last two bits of the slot number. | The ROM pages ($3C-$3F) can be written to any of the eight available MMU slots. In all cases the last two bits of the page are ignored by the MMU and substituted by the last two bits of the slot number. | ||
This essentially means that page $3C will only appear in slots 0 and 4; $3D in 1 and 5; $3E in 2 and 6; $3F in 3 and 7. | This essentially means that page $3C will only appear in slots 0 and 4; $3D in 1 and 5; $3E in 2 and 6; $3F in 3 and 7. | ||
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Home / Articles - MMU RAMROM Mode
Color Computer 3 Memory Management Unit in ROM/RAM
The ROM pages ($3C-$3F) may be written to any of the eight MMU slots. However, unlike RAM pages, the MMU ignores the lower two bits of the value written when the page refers to ROM. Instead, it replaces those bits with the lower two bits of the slot number.
For example, writing $3D to slot 2 of the MMU does not map the second 8k page of ROM to addresses $4000-$5FFF. It maps page $3E (the third ROM page), because:
In ROM/RAM mode there are three different ROM configurations available. They are selected by setting bits 1 and 0 of address $FF90 to:
| $3C | $3D | $3E | $3F | Bit Value |
|---|---|---|---|---|
| Extended Color BASIC | Color BASIC | 16K Cartridge ROM | 0X | |
| Extended Color BASIC | Color BASIC | Reset Initialization | Super Extended Color BASIC | 10 |
| 32K Cartridge ROM | 11 | |||
| CPU Address Range | Slot Address |
|---|---|
| $0000 - $1FFF | Slot 0: $FFA0, $FFA8 |
| $2000 - $3FFF | Slot 1: $FFA1, $FFA9 |
| $4000 - $5FFF | Slot 2: $FFA2, $FFAA |
| $6000 - $7FFF | Slot 3: $FFA3, $FFAB |
| $8000 - $9FFF | Slot 4: $FFA4, $FFAC |
| $A000 - $BFFF | Slot 5: $FFA5, $FFAD |
| $C000 - $DFFF | Slot 6: $FFA6, $FFAE |
| $E000 - $FFFF | Slot 7: $FFA7, $FFAF |
The ROM pages ($3C-$3F) can be written to any of the eight available MMU slots. In all cases the last two bits of the page are ignored by the MMU and substituted by the last two bits of the slot number.
This essentially means that page $3C will only appear in slots 0 and 4; $3D in 1 and 5; $3E in 2 and 6; $3F in 3 and 7.
